The invention relates to wide mat-type conveyors made up of links or modules connected together in chain-like fashion and driven by a drive shaft and sprocket assembly. More specifically, the invention is concerned with an improved arrangement for maintaining the conveyor belt in proper alignment with the conveyor frame despite fluctuations in belt width caused by drastic changes in temperature.
U.S. Pat. No. 4,438,838 discloses a chain link conveyor belt formed of a plurality of pivotally interlocking and substantially rectangular modules. Both interlocking ends of each module include a series of spaced link ends having central apertures to permit the insertion of a pivot pin which joins adjacent modules in interlocking fashion. Each module also includes a support portion which connects the two sets of link ends of the module and which is used for conveying loads. Both the width and the length of the belt may be varied by changing the number and/or the width of the modules.
The conveyor link modules can be made of a variety of materials, but they are preferably made of any one of various types of plastic for reasons of cost efficiency, weight reduction and corrosion resistance. Also, this type of conveyor is commonly used in processing facilities in which products are conveyed from one processing step to another. Often, these processing steps entail drastic changes in environmental conditions such as temperature and humidity. Fluctuations from room temperature to -50.degree. F. or from room temperature to 212.degree. F. are not uncommon. These extreme variations in the conveying environment cause changes in conveyor dimensions due to the expansion and/or contraction of conveyor modules. Consequently, the materials used in the conveyor support and drive apparatus must be capable of both withstanding extreme environmental fluctuations and adapting to changes in conveyor belt dimensions.
U.S. Pat. Re. No. 30,341 to Lapeyre discloses a conveyor sprocket and drive shaft assembly in which the central sprocket is locked to a drive shaft, while the remaining sprockets are free to move axially in either direction to adapt to fluctuations in chain width. Although this design appears to solve the problem of fluctuations in conveyor width, in many applications this arrangement is unsatisfactory. This is due in great part to the fact that the Lapeyre sprocket and drive assembly is "especially designed for use with a chain link conveyor belt of the type described in the aforementioned U.S. Pat. No. 3,870,141 . . ." (Re. 30,341, col. 2, lines 29-31).
The conveyor belt disclosed by Lapeyre is formed of modules provided with relatively narrowly spaced elongate members connecting sets of link ends. The link ends of sequential modules are connected in interlocking fashion by an elongate pin to form a conveyor belt as described above. The narrow spaces between adjacent elongate members serve as sprocket tooth pockets for the teeth of the conveyor drive sprockets, allowing each tooth to drivingly engage a link end of the adjacent connected conveyor module. Thus, the teeth of the single locked drive sprocket will engage the belt between the same two elongate members of the centrally-disposed modules with each cyclical rotation of the belt. The two elongate members as well as the sprocket teeth will be subject to wear due to bi-directional, laterally-directed loads caused by expansion and contraction of the belt as well as by uneven belt loading, shaft misalignment and other inaccuracies in conveyor manufacture. The relatively narrow spacing between adjacent elongate members, coupled with the close fit of the sprocket teeth between the members, does not accommodate significant lateral variation of the belt alignment before sprocket/belt meshing problems are encountered. When the conveyor belt is made of plastic, as is the case with the preferred Lapeyre embodiment, the elongate members and particularly the sprocket teeth are particularly subject to failure caused by this combination of wear and lateral misalignment.
In addition, Lapeyre teaches the use of a pair of C-clips to secure the locked drive sprocket to the shaft so as to prevent axial movement thereon. One clip is placed on either side of the sprocket, and the clips retain the sprocket on the shaft by means of grooves cut into the corners of the shaft. It is common engineering knowledge that the cutting of such grooves into a shaft will weaken the shaft. Conveyor drive shaft strength is often critical, due to the amount of loading of the conveyor belt, and this principle applies to conveyors made of plastic as well as metal.
Lastly, it is time consuming to assemble and maintain an extra-wide conveyor drive when C-clips are used to retain the sprockets, and C-clips are often unavailable for shafts having larger cross sections.